British drug policy is not working. This week we have learned that the UK spends 0.48% of GDP on drug prevention – more than any other country in Europe – but has one of the highest levels of drug use, with more than three million users in 2009-10.

Many of us were heartened in 2002 when David Cameron, recognising this problem, argued: "Drugs policy in this country has been failing for decades. Drug abuse has increased massively, the number of drug-related deaths has risen substantially and drug-related crime accounts for up to half of all acquisitive crime." Wise words.

There are clear political sensitivities surrounding drug policies, rendering effective reform a challenging prospect for politicians once they are in government. But most sensible politicians, officials and scientists recognise that 50 years of a criminalising approach hasn't reduced problem drug use. We therefore call on the prime minister to convene an all-party commission to review drug policy and make recommendations for reform.

This week the European Monitoring Centre for Drugs and Drug Addiction found that, of the countries it studied, the UK has the highest proportion of people who have used amphetamines (more than one in 10), the highest proportion of ecstasy users and the second highest of cocaine users. These drugs are clearly harmful.

But in the UK we have already seen that health-led approaches, such as heroin treatment centres, have better outcomes than criminalising approaches. So it is of great concern that our government is continuing with a predominantly criminal justice approach that simply does not work. It is time to look more closely at the scientific evidence.

Since July 2001 drug possession in Portugal has been an administrative offence, rather than a criminal one. Instead of courts and jail sentences, users are sent before commissions which take a holistic approach to drug policy – combining health policies with rehabilitation. The results are important. The prevalence of problem drug use and the number of young people addicted to drugs have fallen. So, too, has the number of users in prison.

Scientific research makes it clear that drug policy should be re-examined. At the same time, public opinion has shifted strongly, enabling drug reform to move into mainstream politics. In a recent study by YouGov, 53% of those polled stated that they did not feel the government's approach to illegal drugs was working. Indeed, only 11% of those asked thought drug policies were in any way effective. At their conference in September the Liberal Democrats passed a detailed policy resolution on drug policy reform.

On the international stage, the UK, along with the US and Spain, are the top drug user countries. We must bear significant responsibility for the consequences of our demand for drugs from Latin America.

In the past five years there were about 40,000 drug-related deaths in Mexico, a significant number of which were children. President Juan Manuel Santos described Colombia as a country nearly broken by a combination of drug cartels and guerrilla narcotics traffickers. Indeed, just this week, the long-term international failures led Santos to call for a global rethink on drugs policy.

This year marks the 50th anniversary of the 1961 UN Convention on Narcotic Drugs. The all-party parliamentary group for drug policy reform is marking this anniversary with a top-level summit of government representatives from 14 countries with senior UK delegates.

The event, taking place this week, will launch the Global Commission Report on Drug Policy in the UK, it will present recent research and country experiences of evidence-based health-orientated policies and it will discuss how to move forward despite the criminalising UN conventions.

The time has come for the UK government to decide what it will do to help promote this agenda. Britain still has the opportunity to be a world leader.